Briquette for producing a foamed slag effect in eaf technology in stainless steel production

ABSTRACT

It is known that a foamed slag can be produced on stainless steel melts in an electric arc furnace by introducing a mixture of metal oxides, limestone, carbon and a binder in the form of briquettes into the furnace so that the briquettes become arranged there in such a way that the metal oxides are reduced by the carbon and the limestone is thermally dissociated at the metal-slag interface beneath the slag and the resulting gases bring about foaming of the slag by bubble formation. This foaming on steels having a high chromium content, due to the significant uptake of chromium oxide by the slag, leads to problems which result from the physicochemical properties of slags having a high chromium oxide content. To provide, according to the invention, briquettes suitable for producing foamed slag having a high chromium oxide content, which is carried out, in particular by the process of WO 2010/003401 A1, all briquette properties to be taken into account and all materials suitable for briquette production are tabulated in the form of a matrix.

The invention relates to a briquette for producing a foamed slag onstainless steel melts in an electric arc furnace, containing a mixtureof metal oxides, limestone, carbon, and a binder which is placed in thefurnace and arranged there such that the metal oxides are reduced andthe limestone is thermally dissociated beneath the slag at themetal-slag interface and the gases formed in this process make the slagfoam by forming bubbles.

In the operation of electric arc furnaces, packed-in solid materials,primarily scrap and alloys, are melted with the electric arcs of theelectrodes, which project from above into the furnace chamber. In thisprocess the slag, aside from its primary function of removingundesirable constituents from the melt, performs a protective function,since it partially fills the space between the electrode end and themetal surface and protects the refractory lining of the furnace from theradiant energy of the electric arc furnace. This protective function ofthe slag can be improved if the slag is made to foam by suitablemethods. The foaming of the slag is one of the decisive processsequences in stainless steel production in an electric arc furnace thataffect the increase in efficiency of steel production and reduce theproduction costs.

Therefore this is a generally employed method in the production ofstandard carbon steels. However, the situation is different when this isto be applied to foams on steels with high chromium content, since thesignificant uptake of chromium oxide by the slag causes problems. Thisis a consequence of the physicochemical properties of slags with highchromium oxide content.

To be sure, methods for foaming slag with high chromium content areknown, but all of them are unsatisfactory. For example in EP 0 829 545B1 a method is suggested for producing a foamed slag on molten stainlesssteel in an electric arc furnace by introducing into the slag, using aninjection medium such as nitrogen, a powder composed of a metaloxide—either zinc oxide or lead oxide—and carbon. The oxide contained inthe powder is reduced when it reacts with the carbon. In this processbubbles are formed in the slag; these which essentially consist ofcarbon monoxide and make the slag foam. Because of the relatively largesurface area associated with the powder form, a brief, vigorous reactionwith the slag occurs, and it takes place in a limited area near thepoint of injection or blowing into the melt bath.

To avoid the drawbacks of introducing powdered substances, it issuggested in WO 2004/104232 that the materials used for foaming theslag, a mixture of metal oxide and carbon, be loaded into the electricarc furnace in the form of pressed molded pieces. The density of thesemolded pieces is adjusted such that they float in the slag, preferablyclose to the foam/slag interface.

To minimize unwanted loss of valuable material in the case of highchromium oxide content in the slag, it is suggested in WO 2008/095575 A1that the pellets or briquettes, composed of a defined mixture of an ironsource as the ballast material, carbon or carbon and silicon as thereducing agent, and a binder, be charged into the electric arc furnacein such a way that they undergo a chemical reduction reaction with themetal oxides of the slag, especially with the chromium oxide present,under the slag layer, wherein the reaction gases produced, primarilycarbon monoxide, support slag foaming.

In addition, WO 2010/003401 A1 describes a method for producing foamedslags with which the foaming of a slag with high chromium oxide contentcan be achieved. The core of this process is that the material containsan iron oxide source, carbon source, high carbon-containing ferrochromeand/or scrap or nickel oxide (other than ferritic steel) as admixturesas well as limestone and possibly additional fluorspar, binders such asmolasses and/or cement, and additional gas sources for the foamingprocess. This material is to be present in briquette form or as pelletswith different sizes. The mixture to be introduced in the form of moldedpieces, such as briquettes or pellets, contains as its basic components

-   -   Iron oxide (Fe₂O₃, Fe₃O₄) in any form [such] as scale, converter        or EAF/LF dry dust, [or] converter wet dust (sludge),    -   Coke, graphite, carbon (C)    -   Ballast material for all stainless steel types in the form of        FeCr, ferritic scrap ballast material for stainless austenitic        and duplex steel types in the form of FeCr, ferritic scrap,        austenitic scrap, duplex scrap, nickel oxides (NiO_(x))    -   Limestone (CaCO₃), lime or fluorspar (CAO or CaF₂)    -   Al oxide (Al₂O₃)    -   Binder in the form of molasses, cement, or another binder.

The goal of the invention is to supply suitable briquettes for theproduction of foamed slags with high chromium oxide content, performedparticularly according to the method of WO 2010/003401 A1, wherein allbriquette properties to be considered as well as all materials suitablefor briquette manufacturing are summarized in the form of a matrix.

The task posed is accomplished with the characterizing features of claim1 in that the briquette for use in different melts, such as stainlesssteel, low alloy steel or high alloy steel, is made up of definedarbitrary mixtures of individual or multiple substances of the followingbasic components:

O₂ carrier

-   -   Dust, sludge or slag with ≧10% FeO/Fe₂O₃,    -   Dust, sludge or slag with ≧1% Cr₂O₃,    -   Dust, sludge or slag with ≧1% MnO,    -   Dust, sludge or slag with ≧1% NiO,    -   Scale with ≧10% FeO/Fe₂O₃        Gas carrier Dust with ≧40% CaCO₃,        Density adjuster    -   Dust with FeCr,    -   Dust with Fe/low alloy fine scrap,    -   Dust with Cr/ferritic fine scrap,    -   Dust with Ni/austenitic fine scrap/grinding dust,    -   Dust with Mn/ferritic or low-alloy fine scrap,        Reducing agent Carbon-containing substances ≧90% C,    -   Dust or fine granulate of coke, coal or graphite,        Binders Molasses, cement, Ca(OH)₂ (slaked lime), in each case        ≦5%.

Advantageous embodiments of the invention will become apparent from thesubclaims.

The use of foaming of the slag in EAF metallurgy provides a number ofadvantages, such as improvement of the thermal efficiency of the furnacebecause of the low thermal conductivity of the foam, low consumption ofrefractory material and electrodes, and stabilization of the electricarc furnace and the noise level.

To achieve effective foaming, a high gas production must be achieved atthe metal/slag interface. The dominant factors here are the gas CO whichserves as the foam generator and CO₂. These gases form during thereduction of iron oxide and chromium oxide and the thermal dissociationof the limestone as follows:

Fe₂O₃+3C=2Fe+3CO  (1)

FeO+C=Fe+CO  (2)

Cr₂O₃+3C=3CO+2Cr  (3)

CrO+C=Cr+CO  (4)

CaCO₃=CaO+CO₂  (5)

In these reactions, the degree of reduction of the iron oxide by thecarbon is very high, while the reduction of chromium oxide by the carbonis less effective.

It should be noted that the slags in the manufacturing of stainlesssteel contain very little iron oxide but a large amount of chromiumoxide, so that the low efficiency of CO generation in such slags isunderstandable. More effective gas generation may be achievable by thesystematic addition of synthetic materials such as scale and limestone.

The relative density of the additives plays an important role for thefoam production, specifically compared with that [relative density] ofthe slag or the metal. It contributes to bringing the gas productionreaction to the slag/metal interface so that the foaming becomes moreeffective and better controllable.

The density can be influenced by the suitable selection of very densematerials (metals), so-called ballast materials such as ferritic scrapand/or ferrochrome, and less dense materials (oxides).

The principal component in the foam formers is iron oxide (Fe₂O₃), withadded carbon as a reducing agent. Thus the following reaction takesplace:

Fe₂O₃+3C=2Fe+3CO  (6)

wherein the foam mixture of Fe₂O₃ and graphite contains 18.37% graphiteand as the remainder, 81.63% Fe₂O₃.

The composition is completed by ferrochrome (FeCr) with high chromiumcontent, ferritic scrap and limestone CaCO₃.

In the case of slags of austenitic steels, nickel oxide may also beadded.

Ferrochrome and ferritic steel make the foam-forming additives heavierbecause of their high relative density. Thus the relative density isbetween the specific densities of slag and metal according to:

ρ slag<ρ material<ρ metal  (7)

In consequence the material is systematically positioned at theslag-metal interface by buoyancy.

In this process it dissolves in the metal bath, increasing the bathweight.

Thermal dissociation of limestone results in production of CO₂, whichsupports foaming, while calcium oxide dissolves in the slag andincreases the viscosity and basicity of the slag. In addition the slagviscosity can also be adjusted by the addition of fluorspar (CaF₂).

The foam former according to the invention consists of basic componentssuch as

-   -   Iron oxides (Fe₂O₃, Fe₃O₄) in any form [such] as scale,        converter or EAF/LF dry dust, [or] converter wet dust (sludge),        ore    -   Coke, graphite, carbon (C)    -   Ballast material for all stainless steel types in the form of        FeCr, ferritic scrap    -   Ballast material for stainless austenitic and duplex steel types        in the form of FeCr, ferritic scrap, austenitic scrap, duplex        scrap, nickel oxides (NiO_(x))

The additives include:

-   -   Limestone (CaCO₃)    -   Lime and fluorspar (CaO and CaF₂)    -   Al oxide (Al₂O₃)

As binders:

-   -   Molasses    -   Cement    -   Or other possible binders.

The composition of the briquette can be specified as follows:

Fe₂O₃, Fe₃O₄ 10-70 in %  C 2-16 in % Ballast material 14-78 in %  CaCO₃0-10 in % CaO, CaF₂ 0-10 in % Al₂O₃ 0-10 in %

The assumptions that follow are used to determine the relative densityof the foam former, wherein Fe₂O_(3,m), m is to be understood as amixture of Fe₂O₃ with graphite.

-   -   The relative density of the Fe₂O_(3,m), is given by the formula:

$\begin{matrix}{\rho_{{Fe}_{2}O_{3}m} = {{\rho_{{Fe}_{2}O_{3}} \cdot \frac{\%_{{Fe}_{2}O_{3}}}{100\%}} + {\rho_{C} \cdot \frac{\%_{C}}{100\%}}}} & (8)\end{matrix}$

-   -    that of the foam former:

$\begin{matrix}{\rho = {{\rho_{{Fe}_{2}O_{3,}m} \cdot \frac{\%_{{Fe}_{2}O_{3,}m}}{100\%}} + {\rho_{Balast} \cdot \frac{\%_{Balast}}{100\%}} + {\rho_{{CaCO}_{3}} \cdot \frac{\%_{{CaCO}_{3}}}{100\%}} + {\rho_{Binders}\frac{\%_{Binder}}{100\%}}}} & (9)\end{matrix}$

-   -    wherein ballast means FeCr or scrap and nickel oxide

%_(Fe) ₂ _(O) ₃ _(,m)+%_(Balast)+%_(CaCO) ₃ +%_(Binder)=100%  (10)

-   -   The CaCO₃ content can be replaced by CaF₂ contents, wherein in        particular it can be assumed that % CaCO₃=0 or CaF₂=0.

The relative density of the foam former can be seen in Table 1 below.

TABLE 1 Relative density of the pure monolithic foam former componentsused for determining the density of the material Component Ferritic FeCr Fe₂O₃ C CaCO₃ CaF₂ FeCr (*) Molasses Cement scrap NiOx Relativedensity, [t/m³] 7.86 7.2 5.3 2.25 2.27 3.18 4.09 0.99 2.9 6.51 6.67 (*)54% Cr—35% Fe—8% C—3% Si

The relative density data shown relate to monolithic material. On theother hand, if the material for foam formation is used in the form ofbriquettes, its relative density is naturally lower.

The briquettes are produced by pressing the material; differentdensities are obtained depending on the percentage composition.

The relative density of slags produced in steel manufacturing is in therange of 2.5 to 3 g/cm³.

A pressed composition that contains Fe₂O₃ and carbon in the mixturementioned in practice has a density of 3.2 g/cm³, while computationallyfor the individual components a density of 4.7 g/cm³ results. A densityof 2.9 g/cm³ was found experimentally for the slag being considered.

From viewpoint of the desired foam forming effect, the relative densityof the foam former should fall in the range of 2.8-6.0 t/m³.

At low geometric dimensions for the additives (pellets or briquettes)the gas is released rapidly, since the total reaction surface is largerin the case of smaller dimensions.

It was already mentioned that the foam-forming mixture should be addedin the form of briquettes or pellets. In this case the briquettes areproduced in a specially designed press. Dimensions of diagonal 20-100 mmand height 15-40 mm have proven advantageous for the briquettes.

The pellets or briquettes can be produced with addition of molasses orcement in a drum before pressing, but other binding techniques, ensuringthat the desired properties in terms of hardness, fracture strength andcompressive strength are achieved, are also possible.

The briquettes produced by pressing have a briquette volume of ≦800 cm³,a diagonal length between 20 and 100 mm, and a height between 15 and 40mm and are variably designed as ellipsoid, hexagonal cuboids, cuboids orcylinders.

The possible compositions of the briquettes that can be produced fromthe preceding statements as well as the materials that can be used aresummarized in Table 2 in the form of a matrix.

According to this, the briquettes for use in foamed-slag production forstainless steel can be made up as desired from all of the listedsubstances 1-26 of the basic components summarized in Table 2.

For use in foamed slag production for low alloy steel or high alloysteel, the briquettes contain as the O₂ carrier only mixtures of dust,sludge, slag or scale with in each case ≧10% FeO/Fe₂O3 and as thedensity adjuster only mixtures of dust with Fe/low alloy fine scrap anddust with Mn/ferritic or low alloy fine scrap.

A limestone briquette additionally usable in all melts contains as thepure gas carrier no substances of the basic component O₂ carrier.

The physical properties of the possible briquettes produced according tothis Table 2 then fall in the following ranges:

-   -   The CO/CO₂ gas evolution under standard state (20° C., 1        bar)≧0.15 Nm³/kg briquette,    -   The briquette density is between 2.5 and 7.0 g/cm³,    -   After 24 hr of drying and at ≦4% moisture content, the        compressive strength of the briquettes amounts to ≧0.5 N/mm² and        the disintegration strength from 2 m onto a 120 mm thick plate        is ≧97%.

TABLE 2 Stainless Low alloy High alloy steel steel steel Limestonebriquette briquette briquette briquette Material: O2 carrier Individualcomponent or in arbitrary mixture with 1-26 1. Dust FeO/Fe₂O₃ ≧ 10% X XX 2. Dust Cr₂O₃ ≧ 1% X 3. Dust MnO ≧ 1% X 4. Dust NiO ≧ 1% X 5. ScaleFeO/Fe₂O₃ ≧ 10% X X X 6. Sludge FeO/Fe₂O₃ ≧ 10% X X X 7. Sludge Cr₂O₃ ≧1% X 8. Sludge Mn ≧ 1% X 9. Sludge NiO ≧ 1% X 10. Slag FeO/Fe₂O₃ ≧ 10% XX X 11. Slag Cr₂O₃ ≧ 1% X 12. Slag MnO ≧ 1% X 13. Slag NiO ≧ % XMaterial: gas carrier, individual component or in arbitrary mixture with14-19 and 24-26 14. Dust limestone CaCO₃ ≧ 40% X Material: densityadjuster Individual component or in arbitrary mixture with 1-26 15. DustFeCr X 16. Dust Fe/low alloy fine scrap X X X X 17. Dust Cr/ferriticfine scrap X X 18. Dust Ni/austenitic fine X X scrap/grinding dust 19.Dust Mn/ferritic or low alloy X X X X fine scrap Reducer: Individualcomponent or in arbitrary mixture with 1-26 20. Carbon-containing dust ≧90% X X X X 21. Coke (dust, fine granules) X X X X 22. Coal (dust, finegranules) X X X X 23. Graphite (dust, fine granules) X X X X Binder:Individual component or in arbitrary mixture with 1-26 24. Molasses ≦ 5%X X X X 25. Cement ≦ 5% X X X X 26. Calcium hydroxide (slaked X X X Xlime) ≦ 5%

1. Briquette for producing a foamed slag on stainless steel melts in anelectric arc furnace, containing a mixture of metal oxides, limestone,carbon and a binder, which is introduced into the furnace and arrangedthere such that below the slag at the metal-slag interface, the metaloxides are reduced by the carbon and the limestone is thermallydissociated and the gases produced in this process cause foaming of theslag by forming bubbles, characterized in that the briquette for use indifferent melts such as stainless steel, low alloy steel or high alloysteel is made up of defined arbitrary mixtures of individual or multiplesubstances of the following basic components: O₂ carrier Dust, sludge orslag with ≧10% FeO/Fe₂O₃, Dust, sludge or slag with ≧1% Cr₂O₃, Dust,sludge or slag with ≧1% MnO, Dust, sludge or slag with ≧1% NiO, Scalewith ≧10% FeO/Fe₂O₃ Gas carrier Dust with ≧40% CaCO₃, Density adjusterDust with FeCr, Dust with Fe/low alloy fine scrap, Dust with Cr/ferriticfine scrap, Dust with Ni/austenitic fine scrap, grinding dust, Dust withMn/ferritic or low alloy fine scrap, Reducing agent Carbon-containingsubstances ≧90% C, Dust or fine granulate of coke, coal or graphite,Binder Molasses, cement, Ca(OH)₂ (slaked lime), in each case ≦5%. 2.(canceled)
 3. Briquette according to claim 1, characterized in that thebriquette for use in foamed slag production for stainless steel is madeup as desired from all substances of the six basic components mentioned.4. Briquette according to claim 1, characterized in that the briquettefor use in foamed slag production for low alloy steel or high alloysteel as an O₂ carrier contains only mixtures of dust, sludge, slag orscale with in each case ≧10% FeO/Fe₂O₃ and as a density adjuster onlymixtures of dust with Fe/low alloy fine scrap and dust with Mn/ferriticor low alloy fine scrap.
 5. Briquette according to claim 1,characterized in that as a briquette additionally usable in all melts,as a pure gas carrier it contains no substances of the basic componentO₂ carrier.
 6. Briquette according to claim 1, characterized in that theCO/CO₂ gas evolution in the standard state (20° C., 1 bar) is ≧0.15Nm³/kg briquette.
 7. Briquette according to claim 6, characterized inthat the briquette density is between 2.5 and 7.0 g/cm³.
 8. Briquetteaccording to claim 6, characterized in that after drying for 24 hr andat ≦4% humidity the compressive strength of the briquette ≧0.5 N/mm² andthe disintegration strength from 2 m on a 120 mm thick plate is ≧97%. 9.Briquette according to claim 6, characterized in that the briquette, thediagonal size of which is between 20 and 100 mm and the height of whichis between 15 and 40 mm with a briquette volume of ≦800 cm³, is designedvariably as an ellipsoid, as a hexagonal cuboid, as a cuboid, or as acylinder.